Noise generated by aeroengines is a significant proportion of the overall noise generated by an aircraft. As more stringent environmental noise pollution legislation comes into effect it becomes more important to reduce noise by previously uneconomic airframe configurations.
One source of noise pollution from the aircraft is noise generated by the engine, which is then reflected from an aerofoil surface of the aircraft such as a fuselage, tailplane or wing. The engine noise may be either from the engine itself or from the exhaust jet.
In the paper, “Model Tests Demonstrating Under-wing Installation effects on Engine Exhaust Noise”, AIAA-80-1048, Way, D. J. & Turner, B. A., 1980, it is recited that reflection of the jet noise from the wing under-surface is evident, but is less than predicted. The reduced reflected noise is believed to arise principally from attenuation by the exhaust jet as the reflected noise passes through the jet exhaust.
Furthermore, in “Wing Effect on Jet Noise Propagations” AIAA-80-1047, 1980, Wang, M. E. acknowledges that the engine noise is reflected by the underside of the wing and part of which is then refracted and attenuated by the jet exhaust. Wang proposes a number of measures to reduce noise enhancement due to wing effects, firstly the jet engine should be positioned so that the major source distribution of the jet noise would be downstream of the trailing edge of the wing and secondly surface treatment of the underside of the wing. However, positioning the engine toward the trailing edge of the wing to avoid reflected noise would cause the engine to receive undesirable airflow from the underside of the wing and suffer an interference drag penalty. Furthermore, an engine mounted rearward of the wing trailing edge would need to be sufficiently far back that in the event of a rotor burst, fragment trajectories would not pass through the wing. Surface treatment of an aerofoil surface would cause a negative impact on the aerodynamic performance of the aircraft.